1. Field of the Invention
The present invention relates to a lead frame which is used for semiconductor devices such as a semiconductor integrated circuit (IC), and particularly to a lead frame which is essentially preferable to a plastic-mold-type IC package.
2. Prior Art
It is conventionally demanded that in a semiconductor device such as an IC, a highly-reliable and low-cost IC-mounting method is developed. In response to a recent tendency in which the circuit-integration scale of the semiconductor IC is enlarged, the IC package which contains it must be designed to provide a large number of pins and consequently a narrow pitch between the pins. However, even advanced precision soldering techniques cannot keep with such accelerated large-scaling tendencies of the semiconductor IC.
In short, when soldering the terminals of an IC having a large number of pins and a narrow pitch between the pins, it is extremely difficult to constantly supply a predetermined amount of solder to the fine soldering portion of the IC. Conventionally, these integrated circuits are mainly soldered by hand.
In such hand soldering, when supplying the solder to the fine soldering portion, there is a drawback that the amount of the solder supplied cannot be fixed. Thus, when the amount of the solder supplied is smaller than the required amount, the soldered Joint portion may lack Joint strength. On the other hand, when the amount supplied is excessive, the adjacent terminals must be connected together by a bridge portion formed by excessive amounts of solder so that a short circuit is formed.
In order to cope with such drawbacks, there is provided another IC-mounting method for mounting the above-mentioned kind of semiconductor device onto a substrate. According to this method, the leads of the IC are subjected to solder plating in advance, for example. Then, when effecting the soldering operation so as to mount and electrically connect the IC to the substrate, the solder provided by the external equipment is supplied to the Joint portion.
In the above-mentioned method, the solder plating which is pre-effected at the terminals of the semiconductor device is employed to improve a leak-free joint against the solder provided by the external equipment. Herein, such solder plating is made in a thickness of some micro-meters. However, such thin solder-plated layer cannot offer sufficient Joint strength. For this reason, the soldering operation is performed by supplying the solder from the external equipment in a manner which compensates for the lack of the Joint strength, in accordance with the following method.
As the supply method for the solder, four methods are provided as follows. In a first method, the solder is supplied by use of thread solder. In a second method, solder paste is painted, in advance, on the pad of the substrate by screen printing. In a third method, solder paste is painted on the pad of the substrate by use of a dispenser. In a fourth method, the substrate is soaked in a bath of the solder.
In the case of the so-called QFP-type IC package having many pins and providing a narrow pitch between the leads, such lead-pitch may be designed to be 0.65 mm or less, for example. In this case, even when employing the foregoing methods, if the amount of the solder supplied is slightly excessive, the foregoing bridge portion made of the excessive solder is formed between the leads after the re-flow (or dissolution) of the solder. On the other hand, if the supplied amount is slightly less than the required amount, adequate Joint strength cannot be obtained. For this reason, it is also quite difficult to supply an accurate amount of the solder to the fine soldering portion.
FIG. 1 illustrates a tool 1 which is used to effect electric-solder plating on a large number of leads 6 (FIG. 2) provided in a QFP-type XC package having a large number of pins. This tool 1 is comprised of an upper frame 3, a lower frame 2 and screws 4. Herein, both of the frames 2 and 3 have a square shape, and the upper frame 3 is made of brass, while the lower frame 2 is made of a non-conductive substance.
As shown in FIG. 2, a main portion 5 of the QFP-type IC package having a large number of pins is provided between the lower frame 2 and the upper frame 3. Herein, the frames 2 and 3 are jointed together by the screws 4 such that leads 6 of the main portion 5 are sandwiched between them. The tool 1 is soaked in a solder-plating bath "A" so that many of the leads 6 are soaked. By connecting the tool 1 to a power supply cathode while connecting a solder ingot 7 to an anode of the power supply, the electroplating is carried out.
When effecting solder-plating on the leads of the QFP-type IC, the arrangement of the leads may lack uniformity at the four corners of the QFP-type IC. This causes the plating thickness of the leads positioned in the vicinity of the above four corners to be larger than that of the other leads. Thus, when mounting the QFP-type IC on a printed substrate, there is a drawback that a short circuit may occur among the leads, positioned in the vicinity of four corners of the IC, by the bridge portion to be formed on these leads. Due to the nonuniform plating thickness of the leads, there occurs another drawback that the mounting strength of the QFP-type IC is unstable. In the case of the IC packages other than the above-mentioned QFP-type IC package, if the pitch between the leads is nonuniform, the plating thickness at such leads may be altered.
Next, the description will be given of another example of a conventional lead frame with reference to FIGS. 3 and FIGS. 4A to 4C. FIG. 3 is a plan view illustrating the general configuration of a corner portion of a conventional lead frame. Herein, plural leads 22 are arranged around a semiconductor-element-mounting-portion 21, wherein tip-edge portions thereof are connected together by tie bars 23. In addition, base-edge portions of the leads 22 are connected with respective sides of the semiconductor-element-mounting-portion 21 by dams 24. The semiconductor element (not shown) is mounted on the semiconductor-element-mounting-portion 21. Then, the semiconductor element is connected with the base-edge portions of the leads 22 by a wire-bonding process; thereafter, a molding process is carried out.
FIG. 4A is a perspective view illustrating a lead frame with which a molding process has been used, wherein 25 designates a mold resin portion in which a semiconductor element is molded. As illustrated in FIG. 4A, plural lead frames are sequentially formed. In this state, a resin-cutting process is carried out so as to remove the unnecessary molding materials from the lead frame; and a dambar-cutting process is carried out so as to cut down the dams 24. In this manner, the state of the lead frame shown in FIG. 4A is changed to the next state shown in FIG. 4B. In such state, a plating process is carried out using solder, tin, rare metal and the like. Next, as shown in FIG. 4C, a bending process is carried out with respect to the leads 22. Thereafter, the lead frame is removed, thus completing the entire process of forming the lead frame.
In the above-mentioned process, the leads 22 are subjected to the bending process after effecting the metal-plating process. Therefore, when bending the leads 22, the plated surface thereof may be contacted with and rubbed by the mold surface, so that flaws and cracks may occur on the leads 22. Such problem spoils the appearance of the lead frame; and it also badly affects the soldering process to be made when mounting the semiconductor element.
In order to cope with such problem, we have proposed a new processing method in which the plating process is carried out after effecting the bending process. However, a conventional lead frame is produced under the precondition that the bending process is carried out after effecting the plating process. For this reason, even in the above-mentioned new processing method, when effecting the bending process, each of the leads must be separated, or each side of the mold resin portion must be electrically separated from the other sides. Therefore, it is required that the power supply must be applied with respect to each lead or leads at each side, which complicates the plating process. For example, when effecting the bending process on the lead frame as shown in FIG. 3, a branch portion "a" must be separated from the leads 22, so that each side must be electrically independent from the other sides of the semiconductor-element-mounting-portion 21. This means that if the stay-type branch portion "a" extending from a peripheral-frame branch portion 26 is not cut down, the bending process cannot be carried out on the leads 22.